German Patent No. DE-OS 44 16 641 discloses a vehicle spring strut having a work cylinder that is divided by a damping piston into a high-pressure work cavity and a low-pressure work cavity--both of which act upon the work cylinder. An oil pump formed by a pump rod and a hollow space defined in a piston rod is fastened to the base of the work cylinder. The movements of the vehicle axle and of the piston rod fastened thereto that are brought about by unevenness in the road surface, for example, actuate this oil pump which constantly delivers oil, controlled by valves, from the low-pressure work cavity into the high-pressure work cavity. The piston is accordingly caused to move out of the work cylinder (i.e., during a pulling stage) until the conical tip of the pump rod produces a fluid connection between the high-pressure cavity and the pump cylinder.
The pressure compensation between the high-pressure cavity and the low-pressure cavity is effected via an outlet opening over which the piston passes as the piston is pushed out of the work cylinder.
Initially, i.e. when the vehicle is stationary and the spring strut is not actuated, the pressure between the high-pressure work cavity and the low-pressure work cavity is equalized. Since the damping valves for the pushing (i.e. when the piston is caused to move into the work cylinder) and pulling stages likewise affect the pressure regulation between the high- and low-pressure work cavities, a sharp drop in pressure disadvantageously occurs in the high-pressure work cavity relative to the low-pressure work cavity during the downward movement of the work piston in the case of a high adjustment of the damping valve. When, in addition, the work piston moves over the regulating area defined by the conical tip of the pump rod, the damping medium can flow freely out of a low-pressure oil reservoir into the low-pressure work cavity and from there into the high-pressure work cavity. This causes an undesirable temporary height gain in the vehicle body. A clear disadvantage with respect to pressure regulation consists in that the excessively rapid drop in oil volume in the high-pressure work cavity can only be regulated gradually, that is, gradually returned to the low-pressure oil reservoir.